Metal alloys are usually in a crystalline state in which the atoms are structured in an ordered and repeating pattern. In contrast, amorphous alloys consist of randomly arranged atoms without any structure or repeating pattern. This can occur when the molten alloy is cooled at a sufficiently high rate to prevent the atoms from arranging into ordered patterns and thus bypassing crystallization. The discovery of the “metallic” glass in 1960 led to a “metallically” bonded amorphous solid with thermodynamic and kinetic properties similar to common silicate glasses, but with fundamentally different mechanical, electronic, and optical properties. (See, W. Klement, et al., Nature 187, 869-870 (1960), the disclosure of which is incorporated herein by reference). Metallic glasses are electronically and optically “metallic” like ordinary metals, and exhibit fracture toughness considerably higher than silicate glasses. Owing to the lack of long-range atomic order and the absence of microscopic defects such as vacancies, dislocation, or grain boundaries, metallic glasses exhibit engineering properties such as strength, hardness, and elasticity that are significantly enhanced compared to conventional metals. The absence of microstructural defects influences their chemical behavior as well, often resulting in improved resistance to corrosion and chemical attack. (See, e.g., W. L. Johnson, MRS Bull. 24, 42-56 (1999); W. L. Johnson, JOM 54, 40-43 (2002); A. L. Greer & E. Ma, MRS Bull. 32, 611-616 (2007); and A. L. Greer, Today 12, 14-22 (2009), the disclosures of each of which are incorporated herein by reference).
The remarkably high strength, modulus, and hardness of iron-based glasses, combined with their low cost, prompted an effort over the last five years to design amorphous steel suitable for structural applications. The alloy development effort yielded glasses with critical rod diameters as large as 12 mm and strengths in excess of 4 GPa. (See, e.g., Lu Z P, et al., Phys Rev Lett 92; 245503 (2004); Ponnambalam V, et al., J Mater Res 19; 1320 (2004); and Gu X J, et al., J Mater Res. 22; 344 (2007), the disclosures of each of which are incorporated herein by reference). These low-cost ultra-strong materials, however, exhibit fracture toughness values as low as 3 MPa m1/2, which are well below the lowest acceptable toughness limit for a structural material. (See, e.g., Hess P A, et al., J Mater Res. 2005:20; 783, the disclosure of which is incorporated herein by reference). The low toughness of these glasses has been linked to their elastic constants, specifically their high shear modulus, which for some compositions was reported to exceed 80 GPa. (See, e.g., Gu X J, et al., Acta Mater 56; 88 (2008), the disclosure of which is incorporated herein by reference). Recent efforts to toughen these alloys by altering their elemental composition yielded glasses with lower shear moduli (below 70 GPa), which exhibit improved notch toughness (as high as 50 MPa m1/2), but compromised glass forming ability (critical rod diameters of less than 3 mm). (See, e.g., Lewandowski J J, et al., Appl Phys Lett 92; 091918 (2008), the disclosure of which is incorporated herein by reference).
Another feature of metallic glasses originating from the lack of crystalline periodicity in the atomic structure is a unique soft magnetic behavior of ferrous-metal glasses. Convincing evidence for magnetic ordering in an amorphous metal was first provided by Duwez and Lin in 1967, who successfully produced an amorphous ferromagnetic Fe—P—C foil. (See, P. Duwez & S. C. H. Lin, J. Appl. Phys. 38, 4096-4097 (1967), the disclosure of which is incorporated herein by reference). Duwez and Lin not only demonstrated ferromagnetism in glassy Fe—P—C, but also unusually soft magnetic properties. Because of the absence of a crystal lattice, the magnetic moment in amorphous ferromagnets is not coupled to a particular structural direction, so there is no magneto-crystalline anisotropy. (See, H. Warlimont, Mater. Sci. Eng. 99, 1-10 (1988) the disclosure of which is incorporated herein by reference). Moreover, since the material is magnetically homogeneous at length scales comparable to the magnetic correlation length, the intrinsic coercivity is small. Consequently, amorphous ferromagnetic cores exhibit soft magnetic behavior characterized by high saturation magnetization, desirable for higher power cores with smaller sizes, low coercivity, low magnetic remanence, and small hysteresis, all of which lead to very low core losses and high efficiencies. Due to their superior soft magnetic properties, amorphous metal alloys have been a topic of high interest and have replaced conventional materials in transformer and inductor cores for applications where high performance is required. (See, R. Hasegawa, Journal of Magnetism and Magnetic Materials, vol. 215-216, June, pp. 240-245, (2000), the disclosure of which is incorporated herein by reference). Additionally, these materials may also have applications in sensors, surveillance systems, and communication equipment. (See, H. Warlimont, Materials Science and Engineering, vol. 99, March, pp. 1-10, (1988), the disclosure of which is incorporated herein by reference). As such, amorphous ferromagnetic components are currently used widely in power electronics, telecommunication equipment, sensing devices, electronic article surveillance systems, etc. (See, R. Hasegawa, “Present Status of Amorphous Soft Magnetic Alloys,” J. Magn. Magn. Mater. 215-216, 240-245 (2000), the disclosure of which is incorporated herein by reference). Amorphous magnetic inductors also find applications in pulse power devices, automotive ignition coils, and electric power conditioning systems. All of these applications are possible because of faster flux reversal, lower magnetic losses, and more versatile property modification achievable in amorphous ferromagnets.
Despite all these promising applications, processing techniques and economic viability of incumbent amorphous alloys have limited their impact in industry so far. The early amorphous ferromagnetic alloys introduced in the 1980s were available only in ribbon form with thicknesses of tens of micrometers, owing to its very limited glass forming ability. These ribbons, commercialized under the trade-name Metglas™, were produced by melt spinning on a copper wheel which resulted in melt quenching at rates of 103-105 K/s. Amorphous cores were produced by concentrically laminating these ribbons around a mandrel forming cores of desired shapes and sizes. Although successful, this process had inherent deficiencies: a laborious and expensive laminating process and a low core-packing density due to air gaps left between the thin foils needed to build up the core, which reduces the overall core efficiency. To overcome these deficiencies associated with thin ribbons, the development of ferromagnetic glasses with more robust glass forming ability has been sought in the recent years. For example, Shen and Schwarz reported a ferromagnetic metallic glass capable of forming bulk three-dimensional amorphous hardware with thicknesses up to 4 mm. (See, T. D. Shen & R. B. Schwarz, Appl. Phys. Lett. 75, 49-51 (1999), the disclosure of which is incorporated herein by reference). Although the new bulk glass formers appeared very promising in overcoming the problems of the early ribbons, they suffered from a deficiency of their own: a low fracture toughness, resulting in difficult handling and early fatigue failure.
Over the last three years, significant effort and resources have been devoted to develop solutions that address the deficiencies of both early ribbon-forming ferromagnetic glasses, as well as those of the latter bulk ferromagnetic glasses. Specifically, using a systematic micro-alloying approach, bulk ferromagnetic alloys capable of forming glasses up to 6 mm in thickness while exhibiting fracture toughness values at least twice as high as those of the early bulk glasses, approaching toughness values characteristic of conventional titanium alloys, were developed. (See, M. D. Demetriou & W. L. Johnson, United States Patent Application 20100300148; and M. D. Demetriou, et al., Appl. Phys. Lett. 95, 041907 (2009), the disclosures of which are incorporated herein by reference). The discovery of tough bulk ferromagnetic glasses constitutes a promising development that can lead to efficient and cost competitive fabrication of ferromagnetic cores with superior soft magnetic performance and adequate mechanical performance for power electronics applications, if the magnetic properties of these alloys can be improved upon.
Accordingly, a need exists for Fe-based alloys with particularly low shear moduli (below 60 GPa) that demonstrate high toughness (notch toughness in excess of 50 MPa m1/2) yet adequate glass forming ability (critical rod diameters as large as 6 mm), and improved magnetic properties.